Telephone transmission system and apparatus therefor



Dec. 1l, 1945. c. w. CARTER, JR 2,390,869

TELEPHONE TRANSMISSION SYSTEM AND APPARATUS THEREFOR Filed OCT.. 2, 1942Patented Dec. 11, 1945 TELEPHONE TRANSMISSION SYSTEM AND APPARATUS:THEREFOR Charles W. Carter, Jr., New York, N. Y., assignor to BellTelephone Laboratories, Incorporated, New York, N. Y., a corporation ofNew York Application October 2, 1942, Serial No. 460,520

(cl. lis-44) 6 Claims.

The invention relates to communication systems and particularly totelephone transmission systems and apparatus therefor.

In telephone transmission it is often desirable to provide additionaltelephone circuits between certain points quickly and economically. Forexample, under emergency conditions, such additional circuits might berequired for providing telephone circuitsv over an alternative routebetween two points because of cable failure on the regular route causedby damage due to bombing, sabotage, floods, tornadoes or other storms,etc. Also, it might be necessary to provide such additional circuits inresponse to a sudden demand for service at a military camp, factory orconstruction project at a location not sufllciently supplied withtelephone circuits, or to relieve overloads on congested long haulroutes until regular equipment may be installed.

In such situations, economy and the quickness in supplying the Servicemay be more important than quality considerations. As the number of suchcircuits which will be required, their locations, and the kind of systemto which application is to be made, may be unpredictable inadvance,.this uncertainty emphasizes the need for simplicity andflexibility of application of the additional circuits. Simplicity isdesirable in order that the circuits may be installed quickly by anyavailable men of comparatively little skill in telephone practice underconditions of stress. Flexibility of application requires that the newcircuits be adapted for application to systems of different types, forexample, to voice frequency and carrier frequency telephone systems, andthat the apparatus required for modification of the existing systems togive the additional service be in a, form permitting easy shipment fromplace rto place. Ordinarily, such points would be in toll telephoneoffices easily accessible to railroads but under some circumstances itmight be necessary to install the new telephone equipment at remote,somewhat inaccessibue points, for example, in manholes, requiring thatthe size and weight of the telephone equipment be limited so that it maybe brought to the point of application by a truck or other smallervehicle and installed without the use of special facilities.

An object of the invention is to increase the number of telepho-necircuits available between two points quickly and economically, withoutincreasing line facilities and without excessive interference betweenthe circuits or greatly reduced transmission volumes.

This object is attained in accordancewith the invention by the provisionof a small, portable apparatus unit containing a small amount oftransmission apparatus, at each terminal of an existing circuittransmitting a sufficiently wide band of signal frequencies, to split upthe latter into two or more' circuits each transmitting a narrowintelligible band within the original signal band. The apparatus unitsare adapted for insertion in the circuit terminals either by patching orcross connection.

In on embodiment applicable to a four-wire toll telephone system,filters are employed in the apparatus unit at each terminal for dividingthe normal speech signal band of a single channel into two channels eachcomprising the lower half of the speech band (frequencies below 1850cycles). The reduced Width speech band in channel A is transmitted overthe system unchanged in frequency whereas the reduced width speech bandin channel B is translated by modulation from its normal location in thefrequency spectrum to the upper half of the speech frequency rangebefore being transmitted, and at the other station is demodulated toreturn it to its normal location.

The Various features and objects of the invention will be understoodfrom the following detailed description when read in conjunction withthe accompanying drawing in which:

Figs. 1 and 2 show schematically the invention applied to a four-wiretoll telephone circuit on a four-wire basis and a two-wire basis,respectively.

At the west station of Fig. 1, the two-wire circuits L1 and L2 of thefour-wire tenninatins circuit TC@` are coupled by the hybrid coil HA andassociated balancing network NA in conjugate relation with each otherand in energy transmitting relation with the two-wire subscriberstelephone line LA, and the two-wire circuits La and L4 of a secondfour-wire terminating circuit TCB are coupled by the hybrid coil HB andassociated balancing network Nn in conjugate relation with each otherand in energy transmitting relation with the two-wire subscriberstelephone line LB.

Similarly, at the east station the two-Wire circuits L1 and L2 of thefour-Wire terminating circuit TCA' are coupled by the hybrid coil HA andassociated balancing network NA in conjugate relation with each otherand in energy transmitting relation with the east subscribers telephoneline LA', and the lines La and L4' of a second four-wire terminatingcircuit TCB are coupled by the hybrid coil HB' and associated bal--ancing network NB' in conjugate relation with each other and in energytransmitting relation with a second subscribers telephone line LB'.

The apparatus unit AW at the west station and the apparatus unit AE at`the east `station are usedv to connect the two four-wire terminatingcircuits at thei two stations to opposite ends of the westto-eastone-way transmission pathEA and the east-to-west one-wayV transmissionpath WA of the intermediate four-Wire toll telephone circuit TL toprovide two telephone channelsA and B in eachdirection between thestations,

The apparatus unit AW at the west station comprises two transmittingbranches TA and TB having their inputs respectively adapted forconnection to the circuit L1 of the four-wireterminating circuit TCA andto the circuit L3, of the fourwireterminating circuit TCB, and theiroutputs connected in parallel to'the'i'nput of a low-pass line lter Ithe output Yof which is adaptedfor connection to the west end'of thepath EA; and two receiving branches RA and RB the outputs; of which arerespectively adapted for connection to the circuit L2 of the four-wireterminating circuit TCA, and to the circuit L4 of the four-wireterminating circuit TCaand the inputs of which are connected in parallelto the 'output of the lowpass line filter 2 the input of which isadapted for connection to the west Vend of the path WA.V

'I'he transmitting branch TA includes aloss pad '3 in its' input and alow-pass filter 4 in its output. YThe transmitting branch TB includes inorder reading from west to east the loss pad 5, the low-pass filter 6,the modulator 1 and the high-pass filter 8..; The receiving branch RAincludes `the low-pass filter {ifin its input and the one-way amplifierI in its output. The receiv- K ing branch RB includes in order readingfrom east to West the high-pass filter`Y I I, the demodulator I2, -thelow-pass lter I3 vand the one-way ,am plilcr I4. Carrier waves aresuppliedv to the m`odulator 'I- of the transmitting branch TB and to thedemodulator I2 of the receivingbranch RB from the oscillator I 5. Theoscillator` I andthe ampliers I Il and I4 are suppliedwith energizingcurrent from the common power supply I6.

' Similarly, the apparatus unit AEat the east station comprises twoYtransmittir1g branches YTA and TB having theirinputsrespectivelyadapted for connection to the two-.wire circuitV L1' in the four-wireterminating circuit TCA' associated with subscribers line LA and thetwo-wire circuit L4 of the four-wire terminating circuit TCBQassociatedwithr the subscribers line LB', and their outputsl connected in parallelto the input of the low-pass` line filter I', the output of-V which isadapted for connection to the east end of the. east-to-west path WA;and'two receiving branches RA' and RB having their outputs respectivelyadapted for connection to the two-wire circuit L2' of the four-wireterminating circuit rTCA' associated with subscribers line LA' and tothe twowire circuit L' of the four-wire terminating circuit TCB'associated With'the subscribers linev LB', and their inputs connected inparallel to theoutput ofv a low-pass line filter 2',v ther input o f.which is adapted for connection to the east end` of the eastfto-westpath EA. I VVVjThe transmitting branch TA vincludes in its input theloss pad 3' andan its output the 10W- pass i'llter 4', and thetransmitting branch TB rincludes in order reading from east to west theloss pad 5', the low-pass lter 6', the modulator put, and the receivingvbranch RB includes in order readingv from west to east the high-passlter II', thedemodulator I2', the low-pass filter I3 and the one-wayamplifier Ill'.V Carrier waves are supplied to the modulator 1 of thetransmitting branch TB' and to the demodulator I2' of the receivingbranch'R' by a common oscillator I5'. Energizing currents aresupplied tothe oscillator I5 and to the amplifiers I0' and I4 by the common powersupply I6'. Y The low-pass channel filters 4, 6, 9 and I3 in theapparatus unit AW at the west station are adapted to pass allfrequencies below 1850 cycles in the voice frequency band, and thehigh-pass channel filters 8 and II in that unit are adapted to pass allfrequenciesin the voice frequency band above 1850cycles. The low-passline filters I and 2 .in the apparatus unit AW are adapted to transV mitall'frequencies below 3600 cycles.

Similarly, the low-pass channel filters 4', r6', 9 and I3 in theapparatus unit AEA at thel east station are adapted to transmit'thefrequencies in the voice frequency range below 1850 cycles, and thehigh-pass channel filters 8 and II'fin that unit are adapted to passfrequencies in the voice frequency range about 1850 cycles. Also, thelowpass line filters I' and 2 in the apparatus unit AE at theeast'station are adaptedto pass all `frequencies'fbelow 3600 cycles.r in `The loss pads 3 fand 5 in the transmitting branches of the apparatusunit at'the west station and theV corresponding loss pads 3' and 5' inthe transmitting branches of the apparatus unit AE at the east station,which may be of the adjustable plug type, are providedfor'the purposeYVof maintaining the losses in the transmitting branchesat each stationequaland for'prov'iding level lowering, as needed, to reduce'modulationcrosstalk between channels. The amplifiers and oscillators in theapparatus'units'at the twol sta# tions are preferably of the vacuum tubetype. The

power supply I6, I6', ywhich may be designed to Vto the modulator anddemodulator at each station. f

Assuming that `the apparatus unitsV AW' and AEfat the two stations Yareconnected through tothe associated four-Wire terminating circuits andthe incoming and outgoing klines'EA and WAV of I the Vcircuit TL, thesystem of Fig. l operates asV follows:

Speech waves received at the west terminal station over the subscribersline LA will be impressed by the associated hybrid transformer HA on thetwo-wire circuit L1 of the terminating circuit TCA from whicnit will betransmitted into the transmitting branch TA of the apparatus unit AW.The received Vvoice waves will be transmitted in the latter branchthrough the loss pad 3 and then through the low-pass filter 4 which willlimit the transmitted speech frequencies to the band below 1850 cycles.The latter. band will pass through the 360o-cycle low-pass line filter lto the West-to-east path EA of the four-wire circuit TL, over which itwill pass to the east station.

At the east station the received voice band will pass through the linefilter 2 and then through the low-pass filter 9 in the receiving branchRA', which will select the frequencies below 1850 cycles passed by thelineiilter. These frequencies will then be transmitted to the amplifierl which will amplify them sufficiently to compensate for the lossintroduced by the preceding transmission equipment in channel A for thewest-to-east direction. The amplified voice frequencies will then beimpressed by the hybrid coil HA on the line LA' over which they will betransmitted to and heard by the east subscriber associated with thatchannel.

The voice frequency band of the message wave received at the weststation over the subscribers line LB will be impressed by the hybridcoil HB on the circuit L3 from which it will be transmitted into thetransmitting branch TB of the apparatus unit AW. In the transmittingbranch TB the voice frequency band will be transmitted through the losspad 5 and then will pass to the low-pass filter 6 which will restrictthe transmitted speech frequencies to the band below 1850 cycles. Therestricted frequency band will be modulated in the modulator 'l with thecarrier wave of frequency 3700 cycles received from oscillator l 5. Ofthe modulation products in the output of modulator 1, thefrequencies-above 1850 i cycles will be selected by the high-pass filter8 and the selected frequencies transmitted to the 3600 cycle cut-offlow-pass line filter i which will suppress the upper side-band of theimpressed frequencies and will transmit the lower side-band occupyingthe frequency range from 1850 to 3600 cycles, to the west-to-east lineEA.

At the east station, the received 1850 to 3600- cycle band will passthrough the 3600-cycle lowpass line filter 2 and will be selected by the1850- cycle cut-off high-pass channel filter Il in the receiving branchRe'. I'he 1850-3600 cycle band in the output of the high-pass filter ilwill be combined in the demodulator l2 with the carrier wave of 3700cycles from the oscillator I5'. The lower side-band of the resultingmodulation products will be selected bythe 1850-cycle cut-off low-passfilter i3 and will be amplified by the amplifier I4 to compensate forthe loss produced in the waves by the preceding transmission equipmentin channel B. The amplified band occupying the lower half of the Voicefrequency range below 1850 cycles will then be passed over the twowirecircuit L3' through the hybrid transformer HB to the subscribers line LB over which it will be transmitted to the east subscriber associatedwith channel B. v

The operation of the system for the speech waves in the oppositedirection from the subscribers line LA at the east station to thesubscribers line LA at the west station and from the subscribers lineLB' at the east station to the subscribers line LB at the west station,is identical with that which has been described for transmission in thewest-to-east direction for the similar apparatus for the latterdirection.

Fig. 2 shows the application of the circuits of the invention forobtaining two low quality telephone channels in each direction over afourwire telephone circuit on a two-wire basis. 'I'he apparatus unit AWat the west station includes a combined transmitting-receiving branchTRA and a combined transmitting-receiving branch TRB adaptedrespectively for connection on one side through telephone lines (notshown) to the transmitting and receiving telephone apparatus oftelephone subscribers for the A and B channels, respectively, and on theother side in parallel to one side of the S600-cycle cut-off low-passline filter il. The other side of line filter Il is adapted forconnection to the line lterminals of the hybrid coil Hw at the west endof the fourwire toll telephone circuit TC including the oppositelydirected one-way repeating paths RE and RW.

The branch TRA of apparatus unit AW includes a loss pad iB and thelow-pass lter i9 having an upper cut-off at 1850 cycles, and the branchTRB includes a low-pass filter having an upper cut-off at 1850 cycles, ahigh-pass filter 22 having a lower cut-off at 1850 cycles, and an fintermediate modulator-demodulator 2l supplied with a carrier wave of3700 cycles from the oscillator 23 energized from the power supply 24.Similarly, the apparatus unit AE at the east station comprises atransmitting receiving branch TRA' and a transmitting receiving branchTRB', respectively adapted for connection on one side throughsubscribers telephone lines (not shown) to the transmitting andreceiving telephone apparatus of the subscribers associated withchannels A and B, respectively, and on the other side in parallel to oneside of the 3600- cycle cut-off, low-pass line filter l1', the otherside of which is adapted for connection to the line terminals of thehybrid transformer Ha at the east terminal of the four-wire tolltelephone circuit TC. The branches TRA and TRB include the sametransmission apparatus as the corresponding branches at the west stationas indicated by the use of similar characters for identifying theindividual pieces of apparatus at the two stations, except that at theeast station the characters are followed by a prime mark. Because eachbranch of the apparatus unit AW', AE' at the stations must transmit inboth directions, no one-way amplifiers can be used in those branches.Therefore, in addition to the usual one-way amplifiers (not shown) intheoppositely-directeci one-way paths RE and RW of the intermediate tolltelephone circuit, a one-way amplifier A1, A2 is inser-ted in each ofthe latter paths to make up the losses of the transmission apparatus inthe added apparatus units for the two directions of transmission. lf thevolume loss is not serious a two-wire line can be used instead of thefour-wire circuit. The operation of the system of Fig. 2 for onedirection of transmission will now be describedl The speech frequencyband of the talking currents of a west subscriber having histransmitting and receiving apparatus connected directly or through aswitchboard to the transmission branch TRA of the west apparatus'unitAW' will first pass through the loss pad I8 provided to adjust the lossof the branch TRA to make it approximately equal to that of the branchTRB which includes more transmission apparatus, and to provide 4thelevel lowering required to reduce the modulation crosstalk which occursin the wide band intermediate four-wire circuit TC. The waves then passthrough the low-pass filter I9 which limits the transmitted vfrequenciesto the band below 1850 cycles, which will pass through 360o-cyclecut-off low-pass line filter I1 and will be impressed by hybridtransformer Hw on the West-'to-east repeating path RE ofthe four-wire`toll circuit TC. As previously pointed out, the additional amplifier A1in path RE will make up the additional loss introduced in the path* ofthe transmitted waves bythe transmission apparatus in AW'. f f l j Theamplified limited frequency speech band inthe output of RE will passthrough the hybrid Vtransformer HE to the 3600-cycle cut-ofi' low passline filter Il of apparatus unit AE and willbe transmitted through thatfilter. @The limited frequency Vband `speech waves passed by the linefilterllV will be selected by the 1850-cycle cut-off low-pass filter fI9 Vin branch TRA and willpass through the loss pad I8 utilized for apurpose similar to that of the loss pad I8 in the corresponding branchatY the west station, to a circuit (not shown) leading to the receivingapparatus of the east subscriber associated With'that channel. l

The frequency band of the speech waves of a second westtelephonesubscriber is transmitted tothe transmission branch TRB of the apparatusunit AWf at the west station and will be limited to the frequenciesbelow 1850 cycles by the lowpass filter 20 in that branch. The limitedvoice frequency band will be combined in the modulator-demodulator 2lwith thecarrier wave of 3700 cycles supplied by the associated carrieroscillator 23.

Of the products of modulation the frequencies above 1850 cycles will beselected by high-pass filter 22v and the selected frequencies will betransmitted kto the 3600-cycle' low-pass rfilter I1 which will suppressthe upper side-bandof the impressedv frequencies vand will` transmit thelower side-band occupying the frequency range from 1850 to 3600 cycles.This lower side-band will pass to the hybrid transformer YHw which willimpress it on the repeating path RE of the four-wire toll circuit, inwhich it will receive the required'additional amount of amplification inamplifier A1 and then Willpass through hybrid transformer Hn'and theline fllter l1' of the east apparatus unit AE. 'I'he voice band'1850 to3600 cycles in the output vof filter l1 Vwill then be selected by thehigh-pass filter 22' in transmission branch TRB of that unit and will becombined in modulator-'demodulater 2 I with the 3700-cycle carrier wavesupplied from-oscillator 23,'. Of the products of demodulation the 1850-cycle cut-off, low-pass filter 20 will suppress'the upper side-band andtransmit the lower sideband which will then be transmitted over theassociated circuit (not shown)v to the receiving apparatusv of the eastsubscriber associated with the transmission branch 'I'RB' and' will beheard by him. y e

The operation of the system of Fig. 2 for the two vchannels in thedirection from east'to west is identical with that forrthe west-to-eastdirection as described above. As noted above, in Fig. 2 thefour-Wire'circuit may be a two-wire circuit if wide enoughand if theloss of volume is not serious.

Each apparatus unit of a'system such as shown in Fig. 1, which has beenbuilt and operated satisfactorily, had outside dimensions'of 19 inchesby`12 inchesr by 9 inches, and weighed less than 40 pounds.

Improved operationl of the systems of Figs. l and 2 maybe attainedwithout increasing the cost, size or weight appreciably, by the additionof a voltage limiting device, such as a peak chopper, ofany of theAwell-'known types, in the transmitting branch of each apparatus unit,for

the purpose of further minimizing crosstalk be- 4tween channels. Y

Although the invention has been described only as applied to four-wirevoice frequency circuits it is apparentV that it is applicable Valsoltoany four-wire carrier `telephone circuit transmitting the frequenciesbelow 3600 cycles with realsonable fidelity. Although it was primarilydeveloped for splitting up the frequency-range of ,single existingltelephone channels into two channels of approximately 'half thefrequency range of the single channel, itis apparent that its principlesmay be applied to obtain three or more narrow band channels inreachdirection within the frequency range of the system with which it isused, if that range is'sufliciently wide 1 to allow the frequency widthof the several channels to be made such as to provide thedesired amountof intelligibility.Y

f It is tobe understood that the values which have been given for thefrequency ranges of the filters and the frequency of the carrier wavesapplied to the modulators and demodulators in the systems illustratedand described, are by-wayv and shifted in frequency range as in thecaseof the 3700-cycle carrier. In both cases', the

crosstalk will be unintelligiblebutfthe decreaseV in intelligibilityprovided by the inversion plusV frequency shift favors ythe use of ltheS700-cycle carrier. f

Various other modifications of the circuits. ilf lustratedY and'described which are within thev y spirit and scope ofthe invention willbe apparent to persons skilled in the art.

What is-.claimed is:` Y 1. An apparatus unit for attachment to atelephone line or speech channel for securing two speech channels oflowered quality for emergency. use, said unit having a rst pair of tel;-

minals for connection to said linefor channel and having a second'pairand a third pair oi.' terminals for respective connection to a diiierentone of two telephone instruments Vor. lines, said second pair ofterminals having connection within the unit through a low-pass filter tosaid first pair of terminals, said third pair of terminals havingconnection within the unit through a low-pass filter, a frequencyshifting means and a high-pass filter in tandem, to said first pair ofterminals, each of said low-pass filters having a pass-band suflicientto embrace the lower speechV frequencies essential for 'intelligibilitysaid frequency shifting means'serving to shift the frequency of theWaves from the pass range of one of said two tandem connected filtersbetween said first and third pairs of terminals to the pass range of theothertandem jconnected filter.

2. Anapparatus unit according to claim 1, in whichy the circuitconnection betweensaid first pair and saidsecond pair of terminals is atwothird pair of terminals is a two-way transmission circuit, saidfrequencyishifting meanscomprising a bilateral modulator and a source ofcontinuous frequency carrier waves therefor.

3. An apparatus unit for connection to the transmitting channel of afour-wire telephone circuit for securing two speech transmission pathsof substantially half the frequency width of the speech band which saidchannelis adapted to transmit, said unit having a first pair ofterminals for connection to said channel and a second pair and a thirdpair of terminals for respective connection to a different one of twoincoming speech circuits, said second pair of terminals havingconnection Within the unit through a lowpass filter to said first pairof terminals, said third pair of terminals having connection within theunit through a low-pass filter, a modulator and a high-pass filter inthe order named to said first pair of terminals, each of said low-passfilters having a pass range substantially equal to half the speech bandwidth which said channel is adapted to transmit, said high-pass filterhaving a lower cut-off frequency just above the upper cut-off frequencyof said low-pass filters, and means to supply said modulator with waveshaving a frequency of such value as to raise the frequencies of theincoming waves, in effectively passing through the modulator, from thepassband of the last-mentioned low-pass filter into the pass-band ofsaid high-pass filter.

4. An apparatus unit for connection to the ree ceiving channel of afour-wire telephone circuit for securing two speech transmission pathsof substantially half the frequency width of the speech band which saidchannel is adapted to transmit, said unit having a first pair ofterminals for connection to said channel and a second pair and a thirdpair of terminals for respective connection to a different one of twooutgoing speech circuits, said second pair of terminals havingconnection within the unit through a lowpass filter to said first pairof terminals, said third pair of terminals having connection within theunit through a low-pass filter, a demodulator and a high-pass filter inthe order named to said first pair of terminals, each of the low-passfilters having a pass range approximately equal to half the speech bandwidth which said receiving channel is adapted to transmit, saidhigh-pass filter having a lower cut-off frequency just above the upperycut-off frequency of said low-pass filters, and means for supplying saiddemodulator with waves of such frequency as to lower the frequency ofapplied speech waves received from said one pair of terminals throughsaid high-pass filter, in effectively passing through said demodulator,from the pass-band of said high-pass filter into the pass-band of saidlow-pass filter.

5. An apparatus unit for attachment to each terminal of a four-wiretelephone circuit to provide two telephone channels in each direction,said unit having two pairs of terminals for connection respectively to adifferent side of said four-wire circuit, and having four other pairs ofterminals two of which are for respective connection to a differentreceiving telephone instrument or line and the other two for respectiveconnection to a different transmitting telephone instrument or line, oneof the first two and one of the other two of said four pairs ofterminals having connections within the unit through an individualfilter passing only the lower frequencies essential for intelligibilityin the speech frequency band, to a different one of the first-mentionedtwo pairs of terminals, the other of said first two and the other ofsaid other two of said four pairs of terminals having individualconnection with- Ain the unit through a filter passing only said lowerspeech frequencies, a filter passing only the higher speech frequenciesin the speech band, and an intermediate frequency shifting means intandem, to a different one of said first-mentioned two pairs ofterminals, the order of connection of the tandem filters being reversedin the latter two connections, the intermediate frequency shifting meansin one of said latter connections serving to shift the frequency ofwaves received through the preceding filter of lower speech frequencypass range to the pass range of the succeeding filter of higher speechfrequency pass range, and the intermediate frequency shifting means inthe other of said latter connections serving to shift the frequency ofwaves received through the preceding filter of higher speech frequencypass range to the pass range of the succeeding filter of lower speechfrequency pass range.

6. In combination with a two-way telephone system including stationsconnected by a fourwire telephone circuit having oppositely directedone-way transmission paths for transmitting telephone signals inopposite directions between said terminals each adapted to transmit awide band of speech frequencies, means to secure two speech channels oflowered quality in each direction over said system, comprising anapparatus unit at each terminal comprising two transmitting circuitbranches and two receiving circuit branches, the two transmittingbranches being adapted for connection in parallel to the outgoingone-Way path of said four-wire circuit, the two receiving branches beingadapted for connection in parallel to the incoming one path of saidfourwire circuit, one of said transmitting branches and one of saidreceiving branches being adapted to transmit substantially the same halfof said wide band of frequencies, the other of said transmittingbranches and the other of said receiving branches respectively includingin their inputs one filtering means to select from the applied Waves anarrow band of frequencies Within said wide band, of substantially thesame width as the frequency band transmitted by said one transmittingbranch and said one receiving branch, modulating means to shift theselected band to a different position in the frequency spectrum withinsaid wide band and other filtering means in their outputs forrespectively selecting the shifted frequency band, the width of thenarrowfrequency band transmitted by said one transmitting and said onereceiving branch and that selected by said one filtering means in theother transmitting and receiving branches being such as to embracesufficient essential speech frequencies for intelligibility.

CHARLES W. CARTER, JR.

